This invention relates generally to magnetic recording, more particularly to magnetoresistive (MR) read heads, and most particularly to methods and structures for controlling the stripe height of the MR read heads. Those familiar with the art consider anisotropic magnetoresistive (AMR) read heads, giant magnetoresistive (GMR) read heads, and spin valve read heads to be included in the broader category of MR read heads. Subsequent reference to MR read heads is understood to encompass AMR, GMR, and spin valve devices. Merged inductive write, MR read heads comprise a specific exemplary application in all embodiments described in this invention.
Magnetic disk drives are used to store and retrieve data for digital electronic apparatuses such as computers. In FIGS. 1A and 1B, a magnetic disk drive 10 of the prior art includes a sealed enclosure 12, a disk drive motor 14, a magnetic disk 16, supported for rotation by a spindle S1 of motor 14, an actuator 18 and an arm 20 attached to a spindle S2 of actuator 18. A suspension 22 is coupled at one end to the arm 20, and at its other end to a read/write head or transducer 24. The transducer 24 typically includes an inductive write element with a magnetoresistive read element (shown in FIG. 1C). As the motor 14 rotates the magnetic disk 16, as indicated by the arrow R, an air bearing is formed under the transducer 24 causing it to lift slightly off of the surface of the magnetic disk 16, or, as it is termed in the art, to "fly" above the magnetic disk 16. Various magnetic "tracks" of information can be read from the magnetic disk 16 as the actuator 18 causes the transducer 24 to pivot in a short arc as indicated by the arrows P. The design and manufacture of magnetic disk drives is well known to those skilled in the art.
FIG. 1C depicts a magnetic read/write head 30 including a write element 32 and read element 34. The edges of the write element 32 and read element 34 also define an air bearing surface ABS in a plane 33, which flies above the surface of the magnetic disk 16 during operation.
Read element 34 includes a first shield 44, an intermediate layer 38 which serves as a second shield, and a read sensor 46 located between the first shield 44 and the intermediate layer 38. The read sensor 46 has a particular stripe height, SH, and a particular location between the first shield 44 and the second shield 38, both of which are chosen to attain particular read performance. Control of stripe height is important in controlling device resistance, device output amplitude, device bias point and consequently many related measures of performance. MR sensors can be used with a variety of stripe heights, with a typical SH being smaller than about 2 microns, including less than 1 micron. Further, although the read sensor 46 is shown in FIG. 1C as a shielded single-element vertical read sensor, the read element 34 can take a variety of forms as is known to those skilled in the art, such as unshielded read sensors. The design and manufacture of magnetoresistive heads, such as read sensor 46, are well known to those skilled in the art.
Write element 32 is typically an inductive write element including the intermediate layer 38 which serves as a first yoke element or pole, and a second yoke element or pole 36, defining a write gap 40 therebetween. The first yoke element 38 and second yoke element 36 are configured and arranged relative to each other such that the write gap 40 has a particular throat height, TH. Also included in write element 32, is a conductive coil 42 that is positioned within a dielectric medium 43. As is well known to those skilled in the art, these elements operate to magnetically write data on a magnetic medium such as a magnetic disk 16.
The formation of a read/write head 30 begins with a wafer 50, as shown in FIG. 1D, which includes, formed over a substrate, sets of several layers or films of various materials that form an array of read/write heads (not shown), including the elements of the read/write head 30 that are shown in FIG. 1C. The wafer 50 is then divided into multiple slider bars 52 such that each slider bar 52 has a first cut surface, or edge, 54 and a second cut surface, or edge, 56 substantially parallel to each other. As can be better seen in FIG. 1E, each slider bar 52 may include several read/write heads 60 in series along the bar. For example, a typical slider bar may include about thirty (30) read/write heads 60. As is shown in FIG. 1E, the read/write heads 60 can be of different configuration, however, alternatively each of the write/read heads 60 along the slider bar 52 can be of approximately the same configuration.
As is shown in FIG. 1E, the second cut surface 56 is formed such that the read/write heads 60 extend through to the second cut surface 56. Thus, at the second cut surface 56, the read/write heads 60 are exposed and therefore available for removing material along the second cut surface 56 in a process termed lapping. Alternatively, the read/write heads 60 can extend to near the second cut surface 56, without being initially exposed. In such a case, the read/write heads 60 can become exposed and material can be removed therefrom during the lapping process.
The goal of lapping is to remove material from the second cut surface 56, which defines a lapping plane L, to form the ABS (also shown in FIG. 1C) of each of the read/write heads 60 in the plane 33. More particularly, it is the objective of the lapping process to define the ABS at a precise predetermined distance from the upper edge 64 of the read sensor 46 where the upper edge 64 is defined by wafer processes. In this way, the stripe As height SH of the read sensor 46 (shown in FIG. 1C) is defined substantially orthogonal to the lapping plane L, and the throat height TH is similarly defined substantially orthogonal to the lapping plane L. After lapping, the read/write heads are then each cut from the slider bar to form individual read/write heads.
FIG. 1F shows a typical lapping machine 70. The slider bar 52 is held along the first cut surface 54 by a jig 72. In turn, the jig 72 is contacted by pistons 74 at various bending points 76 along the length of the jig 72. Pistons 74 may be, for example, dual action air cylinders, and are configured to deflect the jig 72 at the bending points 76 by a particular amount. To obtain this particular amount, a controller 78 is used to regulate the operation of the pistons 74. The slider bar 52 is further oriented such that the second cut surface 56 lies substantially parallel to an upper surface 80 of a lapping plate 82. During lapping, an abrasive material, for example a diamond slurry, is introduced between the second cut surface 56 of the slider bar 52 and the upper surface 80 of the lapping plate 82. When the second cut surface 56 is brought into contact or near-contact with the upper surface 80, the slider bar 52 and the lapping plate 82 are moved relative to each other within the plane defined by the second cut surface 56 and the upper surface 80. This movement, along with the forces acting to press together the upper surface 80 and the second cut surface 56 and with the abrasive material placed therebetween, acts to abrasively lap the second cut surface 56 and thereby the read/write heads 60.
Because of the critical nature of the stripe height, SH, it is important to end the lapping process at the particular point which attains the correct stripe height. While lapping times, lapping pressures, and other lapping parameters could be standardized for particular types of slider bars 52, such a method can be ineffective due to fabrication variations such as in the deposition of materials of the read/write heads 60, or the wafer cut locations relative to the read/write heads. More particularly, some fabrication variations may exist within a single slider bar or a single wafer, with variations increasing with distance, while others may exist between different wafers (i.e., wafer-to-wafer variation). Therefore, it is beneficial for the controller to have some indication or feedback of the actual stripe height of the read sensor 46 during the lapping process.
FIG. 2A shows an example of a prior art electrical lapping guide (ELG) 90, that has been used to provide an indication of stripe height during the lapping process. FIG. 2A depicts a slider bar 52 in cross section at a layer including the read sensor 46, and associated leads 92. A "switch" 94, formed of a resistive element, and a resistive element 96 are electrically connected to the controller 78 through the leads 98 and 100, respectively. During the lapping process, a first current I1 passes through the switch 94, and a second current 12 passes through the resistive element 96. As the lapping occurs along the lapping plane L, and while the stripe height, SH, of the read sensor 46 is decreased, the height of both the switch 94 and resistive element 96 are both decreased. Over time during the lapping process, changes in the resistances Rs and Rr of the switch 94 and resistive element 96 respectively, due to the changing heights, can be detected by the controller 78. Such changes in resistance over time are shown in FIG. 2B.
Knowing the material properties and dimensions of resistive element 96 relative to material properties and dimensions of the read sensor 46, the measured resistance Rr during the lapping process can be used to calculate an approximate height of the read sensor 46 during the lapping process. Such a calculated height is shown over time in FIG. 2B by curve 110. Also, the initial height of the switch 94 is chosen such that the entire switch is lapped, thereby "breaking" the switch, before the target stripe height SHd of the read sensor is achieved. Because the height Hs of the switch 94 is known relative to the stripe height SH of the read sensor 46, the remaining stripe height of the read sensor 46 at the time the switch breaks, tsb, can be approximated. Thus, as is shown in FIG. 2B, the height calculated from Rr can be calibrated from the approximated read sensor stripe height at the time the switch breaks, tsb. The lapping process continues until the read sensor 46 stripe height SH is calculated to be approximately the target stripe height SHd at which time, t.sub.end, the rate of the lapping process is changed and ultimately the lapping is ended.
Unfortunately, the switch 94 of the ELG 90 in FIG. 2A does not have a truly digital response, but rather the resistance Rs increases over time as shown in FIG. 2B. Also, measurement circuitry of a typical lapping system cannot easily measure a true open resistance. Thus, there is no easily ascertainable single precise point at which the switch "breaks" or is open, and therefore no precise indication of when the lapping has proceeded to the depth of the switch height for calibration purposes.
In addition, endpoint detection with such an ELG is limited by the measurement precision, with undesirable noise incorrectly indicating that the switch 94 is open, sometimes referred to as a false open indication. One technique to avoid such a false open indication is to place a resistive element in parallel with the switch 94, most typically with a resistance significantly greater than the initial resistance of Rs. The resistance signal detected at the controller will rise to an asymptote equal in value to this parallel resistive element as the switch opens. While a false open indication may be avoided, the resultant endpoint remains imprecise. Furthermore, because such an ELG provides calibration only around the points when the switch breaks, the ELG is ineffective for use in adjusting the lapping parameters throughout the lapping process. Also, the stripe height calculations and calibration of the stripe height calculation using the ELG 90 of FIG. 2A depends on knowing the relative dimensional and material properties of the ELG switch, resistive element, and leads, as well as the read sensor. Therefore, unknown differences in these properties due to fabrication variations can produce incorrect stripe height calculations and therefore incorrect termination of the lapping process, either too early or too late. Such imprecise determination will likely result in a read sensor 46 having an undesired stripe height and therefore substandard performance characteristics. For example, for a typical hard biased contiguous junction sensor, the measured resistance for both switch 94 and resistive element 96, as well as the read sensor 46, will include a leads resistance term and a junction resistance term in addition to the resistance of the switch 94, resistive element 96, or read sensor 46. Each of these terms is unknown because the dimensions of each feature and the sheet resistance of the respective films will vary across any given wafer, as well as throughout a population of wafers. Also, each of these terms is likely to vary with stripe height during lapping. To solve for all of these unknowns for each ELG on a slider bar would require the ability to generate the same number of equations, likely with the same number of calibration switches, and would rely on precise determination of each of the switch endpoints, as well as the measurement precision of each of the total resistance values. To attain such precision would entail an undesirably complex technique, and is therefore impractical.
As a further difficulty, slider bars are known to often have an inherent curvature once cut from the wafer. One objective in defining an air bearing surface is to correct for this condition because this correction is required so as to produce the tightest distribution of stripe height for the read sensors 46 positioned across the slider bar. In a typical embodiment, ELG 90 of FIG. 2A is distributed such that the switch 94 and resistive element 96 are at separate positions between sliders. Due to the unknown curvature of the slider bar, this separation increases the error in assuming that any switch 94 and a neighboring resistive element 96 have precisely known relative stripe heights and therefore results in additional calibration error. Therefore, using ELG 90 is not a satisfactory solution.
FIG. 3A shows another currently used electrical lapping guide (ELG) 120. Such an ELG includes a first resistive element 122 located along the lapping plane L and connected to the controller 78 through leads 124. Also included is a second resistive element 126 electrically connected to the controller 78 through electrical leads 128, but located distantly from the lapping plane L to act as an untapped reference device. A first current I1 and a second current 12 flow through the first resistive element 122 and through the second resistive element 126, respectively, both of which can be measured and monitored by the controller 78 during the lapping process. Further, the dimensions and material properties of the second resistive element 126 are chosen such that, at the point in the lapping process where the stripe height of a read sensor 46 will be equal to the target stripe height SHd, a resistance R1 of the first resistive element 122 is equal to or has some known relationship to a resistance R2 of the second resistive element 126. Thus, during the lapping process, as the stripe height of the read sensor 46 decreases, the height of the first resistive element 122 likewise decreases, thereby changing the resistance R1 measured across the first resistive element 122 as shown in FIG. 3B. Once the resistance R1 is detected to be approximately the same as the resistance R2, as shown in FIG. 3B at point E, the lapping process is stopped at t.sub.end. While only two resistive elements are shown in FIG. 3A, multiple resistive elements can be used. In such ELGs, more than one resistive element can be used as an untapped reference device, providing additional resistance levels with which to determine tend.
Electrical lapping guides such as the ELG 120 of FIG. 3A are also affected by unknown variation in dimensional or material properties across the wafer. The prior art approach of placing a single resistive element between sliders is subject to error both due to dimensional and material properties variation across the wafer and is subject to errors due to bar curvature as previously discussed. Placing the second resistive element 126 in close physical proximity to the first resistive element 122 and designing second electrical leads 128 which are approximately identical to first electrical leads 124 will reduce the cumulative effect of these errors for this structure. Unfortunately, however, in such a modified ELG the dimensional error in defining the stripe height of second resistive element 126 translates directly as an error in targeting the stripe height of first resistive element 124. In effect, the reference during lapping of the bar is not based solely on the position of the upper edge of the respective resistive elements but is also subject to the position of the lower edge of second resistive element 126. Scaling such an ELG to reduce the percentage error in the physical dimensions can inherently invalidate the junction resistance term between the second electrical leads 128 and the second resistive element 126.
Thus, what is desired is an electrical lapping guide and method for controlling the stripe height of a device that is more accurate and results in a more precise determination of the device stripe height substantially throughout a process of lapping a read sensor or other device, while limiting cost and complexity. It is desired that such an ELG would provide a substantially continuous signal such that the stripe height may be determined throughout the process of lapping the read sensor. It is further desired that such an ELG utilize the upper edge of the resistive element as a calibration reference to minimize error. Also, it is desired that such an ELG be substantially insensitive to variation in dimensional and materials properties inherent in processing of the wafer.